Following the standardized high efficiency video coding (HEVC), a HEVC range extension is currently being standardized for developing an extended technique for supporting 4:2:2 and 4:4:4 color samplings and YUV images and RGB images corresponding to 10 bit, 12 bit, and 14 bit depths. Here, unlike an YUV image, a RGB image exhibits high correlations between respective color planes R, G, and B. Based on the same, an extended chroma mode (referred to hereinafter as ‘ECM’) has been suggested by J. Kim at a recent MPEG standardization conference (J. Kim, “RCE1: The performance of extended chroma mode for non 4:2:0 format, JCTVC-M0097, 13th JCT-VC Meeting, Incheon, Korea, April 2013)
The ECM suggested by J. Kim is a technique for predicting pixel values in a chroma (or B, R) area with reference to an encoded luma (or G) pixel in case of an intra prediction for chroma (U, V; B, R in the RGB coding) pixel. The algorithm employs a technique suggested by J. Chen during the standardization of the HEVC, but is not employed by the HEVC (J. Chen, V. Seregin, W-J Han, J. Kim, B. Jeon, “CE6.a.4: chroma intra prediction by reconstructed luma samples”, JCTVC-E266, 5th Meeting, Geneva, Switzerland, March 2011). However, unlike the YUV 4:2:0 format of the HEVC, the RGB 4:4:4 format exhibits high correlations between color planes. Therefore, if the technique suggested by J. Chen is modified in correspondence to the RGB 4:4:4 format instead of the YUV format and applied to the RGB 4:4:4 format, compression efficiency may be significantly improved. In other words, a high coding gain may be obtained. Therefore, the technique may be applied to the HEVC range extension.
However, as a result of analysis of a large number of RGB images, a common RGB image exhibits low correlations between color planes at high frequency areas of the color planes. In other words, due to low correlations between color planes at high frequency areas, high frequency ingredients for luma area may interfere chroma pixel prediction during an application of the ECM, thereby deteriorating compression efficiency.
TABLE 1G/BG/RTest SequenceLLLHHLHHLLLHHLHHTraffic0.9260.6340.6140.5490.9630.8370.8100.777Kimono10.948−0.1050.136−0.0080.977−0.0110.3030.045EBULupoCandlelight0.8200.8590.8690.8990.9040.9440.9290.940EBURainFruits0.9090.9720.9700.9550.8920.9710.9690.956VenueVu0.7200.8850.8530.5530.5970.8400.6040.229DucksAndLegs0.9060.227−0.090−0.1540.9630.2380.125−0.186OldTownCross0.9810.4560.2650.0250.9840.6170.4660.243ParkScene0.9190.2470.4430.1170.9720.4060.4620.126Overall0.8910.5220.5080.3670.9070.6050.5840.391
Table 1 shows results of calculating correlations between four frequency bands between color planes (G-B and G-R) by using first frames of 8 RGB experimental images currently used in the HEVC range extension and shows correlations between color planes corresponding to respective frequency bands in HEVC range extension RGB experimental images. Table 1 is published in “Color plane interpolation using alternating projections,” (B. K. Gunturk, Y. Altunbasak, and R. M. Mersereau), IEEE Trans. Video Processing, Vol. 11, No. 9, pp. 997-1013, September 2002. The technique disclosed therein can analyze frequencies of a RGB image based on mathematical analysis of correlations between color planes R, G, and B, where each correlation has a range from −1 to 1. The higher the correlation indicates the corresponding color planes having the higher correlation to each other.
In Table 1, the LL frequency band indicates low frequency ingredients of an image, the LH and HL frequency bands indicate horizontal and vertical edge ingredients of the image, and the HH frequency band indicates diagonal edge ingredients of the image, that is, high frequency ingredients. Except an EBULupoCandlelight image and an EBURainFruits image, images generally exhibit high correlations in low frequency areas LL and low correlations in high frequency areas HH. Referring to Table 1, RGB images exhibit high correlations (0.90 average) in LL frequency bands and low correlations (0.38 average) in HH frequency bands. As shown in Table 1, high frequency ingredients having low correlations compared to the other frequency ingredients having generally high correlations may deteriorate compression efficiency when pixel values between color planes are predicted by using the ECM.